We propose to continue our studies of the likely reactants and products of biotin mediated carboxylation reactions. Biotin is a vitamin that serves as an obligatory cofactor for a host of carboxylase and decarboxylase enzymes. Our studies will center around the details of half reaction mechanisms currently being proposed for the uptake and transfer of biological carbon dioxide catalyzed by these enzymes. Our principal tools will be high resolution X-ray diffraction and quantum chemical techniques, and these will be employed to study the electronic, vibronic and geometrical properties of reactants and products as well as the most probable reaction pathways that interconnect them. Our goal is to perfect these techniques further and apply them to the biotin questions at large. As a by-product we expect the new approaches to be more robust, simpler to use, and suitable for wider application to biologically interesting problems. The biotin enzymes play pivotal roles in metabolism. Pyruvate carboxylase catalyzes the critical step in gluconeogenesis. Acetylcoenzyme A carboxylase plays that role in fatty acid synthesis. Other biotin dependent enzymes are involved in amino acid and purine metabolism. The preponderance of evidence is that all the biotin enzymes work with a common mechanism. These enzymes show a remarkable degree of genetic conservation, over species and substrates. Therefore, the knowledge we gain from our studies is applicable to a vide body of biochemical processes. Our ultimate goal is to understand the chemistry of the vitamin in detail.